The 300 SLR designation stood for Sport, Leicht, Rennen (Sport, Light, Racing). Despite superficial resemblance to both the 1952 W 194 Le Mans racer and the iconic 300 SL road car, the W 196 S was not derived from either. Instead it was based on the 2.5-litre straight-eight-powered Mercedes-Benz W 196 Formula One championship car, with its engine enlarged to 3.0 litres for sports car racing. All technical work was led by Daimler-Benz motorsport chief Rudolf Uhlenhaut.
The M 196 S engine is a naturally aspirated straight-eight with a bore and stroke of 78.0 mm, yielding 2,982 cc displacement. Peak power output was 310 PS (228 kW) at 7,400 rpm. Like the Formula One unit, the engine was canted 53 degrees to the right to lower the car's profile. It used desmodromic valve actuation — one intake and one exhaust desmodromic valve per cylinder — driven by two spur-gear-actuated overhead camshafts, and Bosch mechanical direct fuel injection. The fuel system ran on standard commercially available Super 98 RON petrol, distinguishing it from the F1 car.
The chassis used a brazed steel tube spaceframe clothed in ultra-light Elektron magnesium-alloy bodywork with a density of 1.8, less than a quarter of iron. Total mass of the roadster was 901 kg. Suspension was fully independent on all four wheels, with torsion bars inside the front frame tubes and a low-roll-centre crossed beam arrangement at the rear to counteract jacking under cornering forces. Large-diameter inboard drum brakes were fitted, too large for 16-inch wheel rims, with short half shafts and double universal joints. At Le Mans, an additional rear-mounted wind-brake (a 0.7 m² light-alloy spoiler) was fitted to reduce wear on the drum brakes, helping compensate for the Jaguar D-type's superior disc brakes.
Nine W 196 S chassis were produced in total: seven roadsters and two closed coupés.
The 300 SLR did not contest the early South American championship rounds. Its first competitive appearance came at the 1955 Mille Miglia, where intense preparation — including multiple familiarisation runs over the public road course in a variety of cars — paid off decisively. Stirling Moss, navigated by journalist Denis Jenkinson using pre-prepared pacenotes, won the 1,600 km race at an average of 157.650 km/h (97.96 mph), setting a record that still stands. Juan Manuel Fangio finished second in a sister car.
At the 1955 Tourist Trophy at Dundrod, the 300 SLRs scored a 1-2-3 finish, decisively denying championship points to rivals. At the 1955 Targa Florio — which replaced the cancelled Carrera Panamericana as the championship's Sicilian round — Moss again led Fangio home in a 1-2 finish, delivering the World Sportscar Championship to Mercedes by two points over Ferrari.
The 1955 24 Hours of Le Mans, however, resulted in the most devastating accident in motorsport history. On the 35th lap, Pierre Levegh's 300 SLR was launched into the spectator area after a collision involving Lance Macklin's Austin-Healey and Mike Hawthorn's Jaguar. The ultra-lightweight Elektron bodywork ignited on impact. Eighty-three spectators and Levegh lost their lives. The leading 300 SLRs were withdrawn from the race during the night. Mercedes-Benz subsequently withdrew from all motorsport at the end of the 1955 season, a decision that had been planned before Le Mans.
In total the 300 SLR won five races, two of them outside the World Championship, and was leading Le Mans when withdrawn after ten hours.
Two of the nine W 196 S chassis were completed as closed coupés with gullwing doors, known today as Uhlenhaut Coupés. These had been set aside by Rudolf Uhlenhaut with the intention of racing them in the Carrera Panamericana, which was cancelled following the Le Mans disaster. When Mercedes announced its motorsport withdrawal, the project was abandoned. Uhlenhaut appropriated one of the completed coupés as his personal company car, fitted with a large muffler to dampen its near-unsilenced exhaust.
With a maximum speed approaching 290 km/h (180 mph), the Uhlenhaut Coupé was the fastest road car in the world in its era. A story that Uhlenhaut drove the 220 km from Munich to Stuttgart in just over an hour circulates widely in motorsport history.
One Uhlenhaut Coupé was preserved by the Mercedes-Benz Museum. Its sibling was sold by RM Sotheby's in May 2022 at the Mercedes-Benz Museum to an unknown private collector for €135 million, the highest price ever paid for a car at public auction or private sale, surpassing the previous record of $75 million for a Ferrari 250 GTO. Proceeds were used to establish the Mercedes-Benz Fund.
The 300 SLR's 1955 championship represents one of the most dominant single-season performances in sports car racing history, cut short by tragedy rather than competition. Mercedes-Benz remained absent from top-level motorsport for three decades following its withdrawal. The Uhlenhaut Coupé's record auction price in 2022 underscored the car's enduring status as one of the most significant objects in automotive history.